Polymeric solar cells (PSCs) and photodetectors have attracted considerable attention in recent years due to their unique advantages of low cost, light weight, solution based processing and potential application in flexible large area devices ((a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J., Science 1995, 270, 1789; (b) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C., Adv. Funct. Mater. 2001, 11, 15; (c) Coakley, K. M.; McGehee, M. D., Chem. Mater. 2004, 16, 4533; (d) Gnes, S.; Neugebauer, H.; Sariciftci, N. S., Chem. Rev. 2007, 107, 1324; (e) Thompson, B. C.; Frechet, J. M. J., Angew. Chem. Int. Ed. 2008, 47, 58; (f) Li, Y. F.; Zou, Y. P., Adv. Mater. 2008, 20, 2952; (g) Yao Y.; Liang Y.; Shrotriya V.; Xiao, S. Q.; Yu, L. P.; Yang, Y., Adv. Mater. 2007, 19, 3979.)
Conventional bulk-heterojunction-type PSCs and photodetectors employ active layers that include a phase-separated blend of an organic electron donor component and an electron acceptor component. Typical donor components are conjugated polymer materials and acceptor components are fullerene derivatives. Ideally the conjugated polymer materials exhibit a high optical absorption coefficient for visible and near infrared (NIR) electromagnetic radiation to provide thin layers of the conjugated polymer materials to maximally absorb incident radiation. Significant effort has been directed to developing donor components having band gaps less than 2.0 eV for solar cell and photodetector applications ((a) Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; and De Boer, B., Polym. Rev. 2008, 48, 531; (b) Perzon, E.; Zhang, F.-L; Andersson M.; Mammo W.; Inganäs; Andersson M. R., Adv. Mater., 2007, 19, 3308). One promising donor component is poly(3-hexylthiophene) (P3HT). The power conversion efficiency (PCE) of PSCs based on blends of P3HT with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) in a thin film has reached about 4-5% under air mass (AM) 1.5 illumination. An external quantum efficiency (EQE) of 76% at 550 nm has been demonstrated from the same blend as a photodetector ((a) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y., Nat. Mater. 2005, 4, 864; (b) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K. H.; Heeger, A. J., Adv. Funct. Mater. 2005, 15, 1617; (c) Schilinsky, P.; Waldauf, C.; Brabec C. J., APL, 2002, 81, 3885). However, the further improvement of P3HT-based PSCs maybe limited due to the incomplete match with the solar spectrum and the large offset of the lowest unoccupied molecular orbital (LUMO) energy levels of P3HT and PC61BM. In addition, because P3HT only utilizes a portion of the solar spectrum, photons of relatively high energy (below 650 nm), P3HT lacks response in the NIR.
To optimize the absorption and band gap of the electron donors, donor-acceptor (D-A) conjugated polymers have been developed. In these polymers, absorption and band gap can be tuned by controlling the intramolecular charge transfer (ICT) from the donor components to the acceptor components. Using this strategy, several low band gap D-A conjugated polymers have been reported showing promising performances with PCE about 3-5% and with photoresponse beyond 750 nm ((a) Mëhlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C., Adv. Mater. 2006, 18, 2884; (b) Wang, E.; Wang, L.; Lan, L.; Luo, C.; Zhuang, W.; Peng, J.; Cao, Y., Appl. Phys. Lett. 2008, 92, 033307; (c) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair E.; Neagu-Plesu, R.; Belletete M.; Durocher, G.; Tao, Y.; and Leclerc, M., J. Am. Chem. Soc. 2008, 130, 732; (d) Hou, J. H.; Chen, H.-Y.; Zhang, S. Q.; Li, G.; and Yang, Y., J. Am. Chem. Soc. 2008, 130, 16144); (e) Yao Y.; Liang Y.; Shrotriya V.; Xiao, S. Q.; Yu, L. P.; Yang, Y., Adv. Mater. 2007, 19, 3979; (f) Perzon, E.; Zhang, F.-L; Andersson M.; Mammo W.; Inganäs; Andersson M. R., Adv. Mater., 2007, 19, 3308).
Conventional D-A conjugated polymers have a linear structure with a main chain that includes alternating electron-rich donor segments (D) and electron-deficient acceptor segments (A), represent by the formula (I):

Device performance of these linear D-A conjugated polymers is highly dependent on their molecular weight, their post-treatments, and the structural regioregularity of the polymers, all of which increases the complexity for preparing these materials and making devices that include these materials. Furthermore, the alternating donor-acceptor structure has been suggested to be responsible for reduced hole mobility because the presence of the electron-deficient moieties on the polymer main chain act as the hole trap and thereby hinder hole conduction along the polymer main chain ((a) Ma, W. L.; Kim, J. Y.; Lee, K.; Heeger, A. J., Macromol. Rapid Commun. 2007, 28, 1776; (b) Ballantyne, A. M.; Chen, L.; Dane, J.; Hammant, T.; Braun, F. M.; Heeney, M.; Duffy, W.; McCulloch, I.; Bradley, D. D. C.; Nelson, J., Adv. Funct. Mater. 2008, 18, 2373; (c) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C., Nat. Mater. 2007, 6, 497; (d) Kim, Y. K.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; Mcculloch, I.; Ha, C.-S.; Ree, M., Nat. Mater. 2006, 5, 197).
Despite the advances in the development of donor-acceptor conjugated polymers for use in active layers of PSCs and photodetectors noted above, a need exists for donor-acceptor conjugated polymers having low and readily tuned band gaps and improved hole mobility. The present invention seeks to fulfill this need and provides further related advantages.